1. Field of the Invention
This invention relates to an improved method for curing an unsaturated polyester resin. More particularly this invention relates to a method for curing an unsaturated polyester resin by adding thereto an organic peroxide, which is a conventional curing agent, and 4-methyl-2,4-diphenyl-pentene-1.
2. Description of Prior Art
Various cured and moulded articles of an unsaturated polyester resin can be obtained by curing and moulding an unsaturated polyester resin with an organic peroxide as a curing agent.
In the curing of unsaturated polyester resins, besides (1) an organic peroxide essentially used as the curing agent, there may be optionally used (2) a curing accelerator and (3) a polymerization inhibitor. The curing reaction of the unsaturated polyester varies with the combination of these additives and the curing temperature can be made to vary in the range of from room temperature up to about 180.degree. C. by suitably altering this combination.
The curing reaction of the unsaturated polyester resin may be generally characterized by the time required for gelation of the unsaturated polyester resin (hereinafter referred to as GT), the time required for complete cure (hereinafter referred to as CT), and the highest temperature reached because of accumulation of the heat generated in the course of the curing (hereinafter referred to as PET).
While GT and CT can be varied by altering the additives, (1) through (3), mentioned above, PET cannot be varied at will.
It is known that PET is desired to be low and that if it is high, then the cured resin acquires high inner stress which may possibly constitute a cause for cracks and deformation.
As reported in Polymer Engineering and Science, Vol. 19, p 774 (1979), if in the compression moulding of a sheet molding compound, PET is high, then the resultant press molded article sustains over cure and consequently fails to exhibit good quality.
As a control to lower PET in the course of curing an unsaturated polyester resin by the addition thereto of a ketone peroxide and cobalt naphthenate, a method involving further addition of t-butylhydroperoxide to the unsaturated polyester resin is suggested in Kunststoffe, Vol. 69, p. 266 (1979).
This method, however, is effective only in curing the unsaturated polyester resin at room temperature in the presence of a ketone peroxide and cobalt naphthenate added thereto and it is not effective in carrying out the same curing at an elevated temperature.
This method, therefore, has only limited commercial value.
The need for developing a curing method for an unsaturated polyester resin to be carried out with PET lowered as desired without reference to the kind of the curing agent added to the unsaturated polyester resin has been keenly felt.